Bistable relays are often used when both switching states, namely switched on and switched off, are assumed for a relatively long period of time. Instead of permanent energization of a coil of the relay in the case of a monostable relay, energization only for the duration of the switchover between the switching states is required in the case of the bistable relay. Within the scope of the disclosure, a relay is generally understood to mean an electromagnetically actuated switching apparatus, i.e. a low-power relay or a DC or AC contactor which is designed for relatively high powers.
In order that a bistable relay assumes a defined switching state on failure of the supply voltage, the bistable relay is often arranged in the series circuit with a capacitor. In the event of loss of supply voltage, the energy stored in the capacitor can be used to bring the relay into a defined switching state, generally into the switched-off state. An electrolytic capacitor is generally used as the capacitor. The circuit arrangement mentioned at the outset can in this case be suitable for a bistable relay in which switching-on or switching-off of the relay takes place via current inrushes of different polarity through an individual coil of the relay, referred to below as the relay coil, or for a bistable relay in which a separate relay coil is provided in each case for the switch-on and switch-off operations.
In order to actuate the relay, it is necessary in each case to apply a minimum voltage to the relay coil for a minimum period of time. Owing to the charging or discharging operation of the capacitor which is connected in series with the bistable relay, a voltage drop which is increasing or decreasing approximately exponentially is set across the relay coil during the course of switching. In order to achieve the situation whereby the minimum voltage provided for the switching operation is present at the relay coil even at the end of the specified minimum time for the current inrush, either a capacitor with a relatively high capacitance needs to be selected or the supply voltage needs to be selected to be so high that the voltage at the relay coil is still sufficiently high even after the preset switching time. A capacitor with a relatively high capacitance is not desirable for reasons of space and costs. Increasing supply voltage is disadvantageous because, under certain circumstances, at least at the beginning of the charging and discharging operation of the capacitor, an excessively high voltage is then present at the relay coil, as a result of which the life of the relay can be reduced.
The document DE 27 47 607 C2 discloses a circuit arrangement for actuating a bistable relay, in which a zener diode is used in order to fix a value of the supply voltage below which a switch-off operation takes place. However, as stated previously, a capacitor with a correspondingly high capacitance connected in series with the relay coil is required. Protection of the relay coil from upward fluctuations in the supply voltage is not discussed.
The document U.S. Pat. No. 4,533,972 also discloses a circuit arrangement for actuating a bistable relay in which a capacitor is provided in a series circuit with a relay coil. Voltage stabilization of the voltage which is applied to the series circuit is provided. In this way, the relay coil can be protected from fluctuations in the supply voltage, but a capacitor with a correspondingly high capacitance connected in series with the relay coil is required in this case too.